The self-assembly of gold nanoparticles (Au NPs) coated with specific organic ions in water was observed by Center for Nanoscale Materials staff in the Nanobio Interfaces, Electronic & Magnetic Materials & Devices, and Nanophotonics groups at the Argonne National Laboratory using in situ transmission electron microscopy (TEM) equipped with a liquid cell. The Au NPs formed one-dimensional chains within a few minutes.

The self-assembly of NPs attracts intense attention for its potential application in the fabrication of hybrid systems with collective properties from different types of materials. The observations provided here clearly elucidate the complex mechanism of charged NP self-assembly processes. They also paint a cautionary tale on using TEM in situ cells to imitate self-assembly processes in actual solution environments.

The CNM's expertise with in situ TEM techniques, colloidal nanoparticle synthesis, zeta-potential measurements, and absorption spectra were exploited in this study.

The hydrated electrons formed in radiolysis of water decrease the overall positive charge of cetyltrimethylammonium (CTA)-coated Au NPs. The NPs also were coated with negative citrate ions. (With citrate alone, however, the Au NPs remained steady in the liquid cell regardless of electron-beam intensity). The anisotropic attractive interactions, including dipolar and Van der Waals interactions, overcome the repulsion among the NPs and induce the assembly of NPs. The spatial segregation of different sizes of NPs as a result of electric field gradients within the cell was observed as well.